Seminars in Cell & Developmental Biology 19 (2008) 161–169
Review of factors essential for blastocyst implantation for their
modulating effects on the maternal immune system
Reproductive Sciences Branch, Center for Population Research, National Institute of Child Health and Human Development,
National Institutes of Health, DHHS, Bethesda, MD 20892-7510, USA
Available online 16 October 2007
Pituitary and ovarian hormones prepare the endometrium for successful blastocyst implantation and support its process directly or indirectly
through the action of growth factors, cytokines and other molecules. Many of the blastocyst implantation essential factors (BIEFs) are modulators
of the maternal immune system. Since little is known as to the action of these molecules on the uterine lymphocytes, its clarification is imperative
to the understanding of the process of blastocyst implantation.
Published by Elsevier Ltd.
Keywords: Implantation; Immune system; Cytokines; Lymphocytes; BIEFs
Hormonal effects on the immune system...................................................................................
Lipid-soluble molecules other than steroids that are essential for implantation .................................................
Uterine receptivity for blastocyst implantation..............................................................................
Calcium and implantation................................................................................................
Trophoblast invasion of the stromal tissue and decidualization ...............................................................
NK cells in pregnant uterus...............................................................................................
Future directions ........................................................................................................
This review will examine the relationship of the factors
identified as essential for implantation (BIEFs) to modula-
ductions, but also those of the maternal immune system. During
the course of pregnancy function of the maternal endocrine sys-
∗Tel.: +1 301 435 6992; fax: +1 301 480 2389.
E-mail address: firstname.lastname@example.org.
the immune environment surrounding the developing embryo
is modified probably due to the molecules derived from the
embryo/fetus/placental unit. Since the embryo visibly develops
nal immune system does not pay much attention to embryonic
development even though it makes for very good immunolog-
ical analysis [1,2]. However, it is essential to study embryonic
development and the surrounding immunological environment
together because the embryo with its surrounding tissues and
immune cells interact with each other throughout the course of
pregnant endometrium may well be influenced by the secretion
of cytokines by endometrial cells in a spatiotemporal manner.
Because of differences in approach between immunology and
1084-9521/$ – see front matter. Published by Elsevier Ltd.
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
endocrinology, there are gaps in the currently available knowl-
edge of the relationship between these two aspects of the same
biological process. It is hoped that this review will assist in fill-
ing these gaps and interdisciplinary differences, so that we may
advance our knowledge on blastocyst implantation sufficiently
to enable us to apply the basic knowledge to alleviate infertility
of women who are suffering from implantation failure.
2. Hormonal effects on the immune system
Hormones influence distribution of leukocytes in the uterus.
Because endocrine condition varies depending on the stage of
pregnancy, the endocrine condition must be clearly understood
before one investigates the uterine immune system. As small
rodents are used in immunological studies, the fact that their
endocrinology is different from humans must be born in our
mind at the beginning of this review. The estrous cycle of
small rodents is short (4–5 days), because this is an incom-
plete cycle, i.e., the luteal phase is missing. After ovulation,
the newly formed corpora lutea are not functional and secretion
of progestins (progesterone and 20alpha dihydroprogesterone)
that increases after luteal formation, declines quickly. Thus, one
may see two peaks of progestin secretion per cycle, one in the
afternoon of proestrus that occurs in response to the ovulatory
surge of LH and FSH and the other on the day of metestrus.
The post-ovulatory progestin secretion is not maintained unless
the corpora lutea are stimulated by twice daily surges of pro-
lactin secretion . Among pituitary protein hormones, effect
of prolactin on lymphocytes is familiar to endocrinologists as
Nb2 lymphoma cells have been used for bioassay and to study
the mechanism of action of prolactin. The presence of prolactin
receptors in the uterus has been known. Prolactin receptors also
exist in the thymus . In humans prolactin is synthesized in
decidual cells and its temporal expression suggests a role in
implantation and placentation . Prolactin stimulates T lym-
phocytes through its receptor [6,7]. However, little is known
about signal transduction of prolactin receptors in lymphocytes.
A recent report  indicates that hCG has direct effects on
the early pregnant endometrium in facilitating implantation of
embryo. hCG induces alterations in expression of genes that
regulate embryo attachment, endometrial remodeling, and the
tocyst in the baboon.
Besides pituitary and extrapituitary gonadotropins, ovarian
steroids influence the immune system, particularly distribution
tion pattern during the estrous cycle and early pregnancy vary
edge obtained in laboratory animals immediately to humans,
one has to check if the steroid levels in the model system
would be comparable to that in humans. Regulation of immune
cells by progesterone has been well known, but the molec-
ular mechanisms involved need clarification. Clemens et al.
 suggested that at least one important role of proges-
terone in maintaining pregnancy is its regulation of cellular
immunity in the maternal–fetal bed. Progesterone has been con-
sidered to protect the embryo/fetus through a molecule named
progesterone-induced binding factor (PIBF) [12,13]. Proges-
terone is considered to act on lymphocytes to release PIBF
that is highly pregnancy-protective by stimulating activities of
type 2 T helper cells (Th2) . There are substantial data
on progesterone–immune cell–cytokine relationship in terms of
pregnancy maintenance. A recent report  shows that pro-
gesterone action to protect maintenance of pregnancy depends
on CD8+ cells because depletion of CD8+ cells by antibodies
caused abortion when pregnant mice were stressed by expos-
ing them to noise. Progesterone acts on uterine CD8+ cells
to produce PIBF which, in turn, tips the balance of cytokines
(Th1/Th2) to increase Th2 which maintains pregnancy. The
Th1 cytokines IFN-gamma and IL-2 can cause fetal loss, while
Th2 cytokine IL-10 is fetal protective . Recent reviews are
referred for further study on the Th1/Th2 paradigm [17,18].
Recruitment of T-regulatory cells is one of the mechanisms by
which tumor cells may foster immune privilege . Differen-
tiation of T cells to T-regulatory cells has also been reported
as an important step for protection of embryo/fetus [20,21].
Foxp3 is a master regulator of Treg cell differentiation .
A reduced expression of Foxp3 mRNA in the endometrium of
for establishment of pregnancy .
Although uterine natural killer (NK) cells contain receptors
for estrogen and glucocorticoid, they do not have progesterone
receptors . However, other studies suggest its presence.
Thus, the molecular mechanism involved in direct action of
progesterone on lymphocytes needs further study. Uterine tis-
sue distribution of macrophages is influenced by ovarian steroid
hormones  and appears to be regulated by receptors [26,27].
Ehring et al.  proposed that progesterone acts on lympho-
cytes as an endogenous immunosuppressant by directly and
reversibly blocking K+channels. While progesterone prevails
in early pregnancy and modulates the immune system, re-
may disrupt the endocrine dominance through interference with
the luteal function . This is an excellent example of the
interaction between the endocrine and immune systems at the
that of estrogen) of progesterone is needed to exert its effect on
the uterus for at least 48h  in a rat in order to obtain bio-
logical effects. One reason may be that progesterone is easily
metabolized in vivo. Progesterone may have a large amount of
target tissues including immune cells. Takabatake et al. 
advanced the implantation window by 1 day by intravenous
injection of the splenocytes from pseudopregnant mice. The
already differentiated under the influence of progesterone, and
to the action of progesterone for 1 day. Thus, these investiga-
tors could make the uterus receptive 1 day earlier by injecting
teristic of the differentiation that was gained by the splenocytes
during pseudopregnancy appears to be worthy of further inves-
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
Precise distribution of subpopulations of lymphocytes in the
uterus, under certain steroid hormone conditions, must be stud-
ied in order to understand the basic interactions between them.
The mechanisms by which ovarian steroids act to differenti-
ate maternal lymphocytes are not fully understood. However,
there are at least two separate biochemical processes have been
reported to be involved in this differentiation. The first report
is on indoleamine 2,3-dioxygenase (IDO) activity and the other
is on programmed death lingand 1 (PDL1). IDO activity pro-
tects the fetus by suppressing T cell-driven local inflammatory
during murine pregnancy resulted in increased rejection rates of
allogeneic concepti but not syngeneic concepti. Fetal rejection
was T cell-dependent but not B cell-dependent. This study pro-
vides evidence that PDL1 is involved in fetomaternal tolerance.
In the same study the investigators tested whether anti-PDL1
antibody may function by enhancing IDO expression, there was
no difference in degree of IDO expression at the maternal–fetal
interface between control and anti-PDL1-treated mice .
In sheep trophoblast cells secrete interferon-tau which plays
an essential role in maintaining pregnancy. Choi et al. 
showed that pregnancy and interferon-tau increase expression
of MHC class I molecules in endometrial stroma and glandular
elongated blastocysts takes place. There was no expression of
MHC class I molecules on the trophectderm of the blastocysts.
The silencing of MHC class I molecule expression on the lumi-
nal epithelium and trophectoderm during implantation appears
critical for protection of the conceptus during this stage of preg-
nancy. Immune-mediated inflammation and allograft rejection
are greatly reduced in certain organs such as eye, brain and
pregnant uterus. This phenomenon is called “immune privi-
lege”. Loss of immune privilege at the feto-maternal interface
results in pregnancy failure. There appears to exist an immuno-
logical mechanism common to these immune privilege sites.
There are a number of molecules found in common in these
immune privileged sites: TGF-beta , FasL , and pro-
teinase inhibitor 9 . TGF-beta has been reported to induce
regulatory T cells, which is essential for the active suppression
that helps evading the maternal immune reaction will serve to
alleviate infertility due to implantation failure .
3. Lipid-soluble molecules other than steroids that are
essential for implantation
Besides steroid hormones, several lipid-soluble molecules
play important roles in blastocyst implantation. Prostaglandins
(PGs) are needed for hatching of blastocyst from the zona
pellucida, spacing of blastocysts along the uterine horn, and
decidualization of the stromal cells at the implantation sites.
Cyclo-oxygenase-2 (COX2), the rate-limiting enzyme in the
biosynthesis of PGs, is essential for blastocyst implantation
and decidualization. Lim et al.  demonstrated that COX2-
derived prostacyclin (PGI2) is the primary PG that is essential
for implantation and decidualization. Several lines of evidence
suggest that the effects of PGI2are mediated by its activation
of the nuclear hormone receptor PPAR?, demonstrating the first
reported biologic function of this receptor signaling pathway.
Production of PGE2is enhanced during inflammation, and this
lipid mediator can dramatically modulate immune responses.
Within the immune system, PGE2modulates the functions of
cell populations, such as T cells and macrophages through
distinct receptors for PGE2. Prostaglandins are metabo-
lites of arachidonic acid, which is generated by enzymes called
lysophospholipases. In this group of enzymes cPLA2alpha was
found to play an important role in “on-time” implantation .
Lysophospholipids are simple phospholipids that have been
recognized as components in the biosynthesis of cell mem-
branes, and their receptors have recently been cloned and the
tion. Ye et al.  demonstrated that lysophosphatidic acid 3
(LPA3) mediated lysophosphatidic acid (LPA) signaling play
essential roles in spacing of preimplantation embryos along
the uterine horns and implantation. These investigators exam-
ined major events from ovulation through decidualization in the
LPA3-deficient mice. There were no effects on ovulation, fer-
tilization and decidualization. However, there was a delay in
implantation and uneven spacing of preimplantation embryos,
clumping towards the cervical end of the uterine horn.
LPA and sphingosine-1-phosphate (S1P) mediate T cell
functions. These lysophospholipids are generated by complex
enzymatic pathways from cell membranes. They protect T cells
specific receptors . The effect on the immune system of the
S1P signaling pathway is noteworthy. The novel immunomod-
ulator FTY720 [a full agonist for S1P receptors] has unique
modes of action such as: (1) a marked decrease in the number of
circulating mononuclear cells, especially T cells, (2) prolong
allograft survival, as this compound modulates the lympho-
cyte chemotactic response to chemokines, and (3) inducing a
homing chemokine-dependent sequestration of lymphocytes to
secondary lymphatic organs, and, thus, preventing lymphocytes
Anandamide is another lipid-soluble molecule that is related
to implantation. Schmid et al.  found very high lev-
els of anandamide in the uterus and that down-regulation of
anandamide levels is associated with uterine receptivity. Mac-
carrone et al.  showed that physiological concentrations
of progesterone stimulate the activity of the anandamide-
degrading enzyme (fatty acid amide hydrolase, FAAH) in
human lymphocytes and reduces anandamide levels, that
the membrane-impermeable conjugate progesterone–BSA was
ineffective in the same concentration range as progesterone
itself, and that progesterone stimulates FAAH activity by
up-regulating gene expression at the transcriptional and trans-
lational level. Further, Maccarrone et al.  showed that
progesterone stimulated the activity of FAAH in human lym-
phocytes up to approximately 270% over the untreated controls.
The activation involved increased nuclear levels of the tran-
scription factor Ikaros. Analysis of FAAH promoter showed an
Ikaros binding site, and mutation of this site prevented FAAH
activation by progesterone in transient expression assays. These
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
studies suggest that binding of progesterone to an intracellular
receptor was needed for progesterone activity. A clinical study
spontaneous abortion . Anandamide inhibits lymphocyte
proliferation  in peripheral human blood. The mouse uterus
contains very high levels of anandamide, particularly when the
uterus is in nonreceptive stage or in the non-implantation sites.
A reduction in anandamide in the uterus is observed during the
receptive stage for blastocyst implantation. Administration of
anandamide agonist inhibits implantation . Thus, aberrant
anandamide production and its receptor expression in the uterus
may lead to pregnancy failure. Possible role of endocannabi-
noid system in the immune response was reviewed by Salzet et
4. Uterine receptivity for blastocyst implantation
In small rodents such as rats and mice there is a distinct
place [30,52]. This receptive period may be induced by steroid
hormone manipulation since it is triggered by a single injec-
tion of estrogen given after at least 48h of progesterone effect.
Because of the distinct timing of onset of the receptive period,
lated with onset of the receptivity. For example, heparin binding
EGF-like growth factor (HB-EGF) is expressed solely at the
apposition sites of blastocysts . Since blastocysts adhere to
the uterine luminal epithelium only at this stage, Pan et al. 
made a microarray analysis of uterine epithelial gene expres-
sion during the window of uterine receptivity. They found that
several of the immune response cytokines such as TNFalpha
and IL-1alpha are downstream of nuclear factor kappaB (NF-
kappaB) signaling. In their study components of the Toll-like
were dramatically up-regulated in the luminal epithelium dur-
dependent and is elevated in non-pregnant estrogenized uterus.
Thus, careful experimental design is needed to look into the
role of this molecule in the implantation process. Nakamura
et al.  investigated whether NF-kappaB is involved in the
process in the mouse. Transient transfection of inhibitory kap-
paBalpha mutant cDNA into mouse uterine lumen , which
lacks signal-dependent phosphorylation sites and cannot disso-
ciate from NF-kappaB, suppressed uterine NF-kappaB activity
to the level less than half of that observed in control on days 3.5
and 4.5 post coitum. After the transfection in the uterus implan-
importance of NF-kappaB in implantation process.
Tranguch et al.  demonstrated that immunophilin
FKBP52 is critical to uterine receptivity for blastocyst implan-
tation. The immunophilin FKBP52 serves as a cochaperone for
steroid hormone nuclear receptors to govern appropriate hor-
mone action in target tissues. However, the localization of this
immunophilin in stromal cells surrounding the blastocyst on
day 5 after implantation, suggests that immunophilin protects
the implanting embryos from immunological rejection by the
maternal immune system. The WT blastocysts transferred into
the uterus of FKBP52(−/−) mice failed to implant while those
transferred into the (+/+) uterus survived .
Stewart et al.  showed that leukemia inhibiting fac-
tor (LIF) plays an essential role in induction of blastocyst
implantation. Expression of LIF is estrogen dependent, but
LIF can replace estrogen effect only partially in induction of
implantation. LIF, COX2 and HB-EGF are all essential for
implantation, these molecules may exert their action on implan-
an AbdominalB-like homeobox gene (Hoxa-10), FKBP52 and
Tranguch et al.  suggested that LIF probably acts on Msx-1
in anandamide. LIF suppresses decidualization of murine uter-
ine stromal cells in the presence of serum  and modulates
trophoblast differentiation .
5. Calcium and implantation
Preimplantation increase in calcitonin production in the uter-
ine glands caused by increase in progesterone suggests that the
luminal epithelium preparation for accepting active blastocysts
may involve calcium homeostasis in the uterine luminal epithe-
lial cells . HB-EGF is expressed by increasing secretion of
progesterone and estrogen in the endometrial luminal epithe-
induced by binding of EGF ligand to its receptor . Wang
et al.  showed that HB-EGF stimulates trophoblast cells
by inducing calcium influx into trophoblast cells supporting
a possible mechanism of EGF–EGFR interaction that relaxes
phoblast between epithelial cells . Li et al.  showed that
E-cadherin expression is reduced in the luminal epithelium of
the endometrium after calcitonin treatment. This suggests that
multiple factors that are related to calcium homeostasis have
an effect on the luminal epithelium for making it receptive to
trophoblast invasion into the endometrium. Progressive expres-
sion of HB-EGF, other members of the EGF family and their
receptors on trophoblast and decidual cells and other tissues of
the endometrium during the process of implantation suggests
a constant interaction between EGF family members and their
receptors at the trophoblast-maternal tissue interface. If inva-
sion of trophoblast cells into the endometrial tissue is indeed
facilitated by influx of calcium into tophoblast resulting in a
reduction of calcium in the maternal tissues, the study of cal-
cium in the interface between trophoblast and the endometrial
tissues including uterine immune cells warrant further investi-
gation. Since recent studies on calcium signaling in lymphocyte
activation suggest that interference with calcium signaling may
be useful approach to prevent allograft rejection in transplan-
tation , reduction of calcium at the embryo–maternal tissue
making the uterine tissues receptive, but also by inhibiting the
immune cells from attacking the implanting blastocyst.
in the peri-implantation mouse uterus . Treatment of
human endometrium with HB-EGF increased cell surface and
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
total decay-accelerating factor (DAF, CD55). DAF is one
of complement-protective proteins that prevent complement-
mediated cell lysis in multiple cell types [71,72]. Young et
al.  postulated that mid-secretory increase in progesterone
secretion stimulates HB-EGF or other members of the EGF
family, which, in turn, stimulates expression of DAF. DAF is a
glycosylphosphatidylinositol-anchored glycoprotein that func-
tions in protecting cells from lysis by autologous complement.
Although DAF knockout mice were able to develop, grow, and
reproduce, showing no overt abnormal phenotypes , this
protein appears to play a role in implantation process as DAF
is expressed in the feto-maternal interface during human preg-
nancy , and on the surface of extra-villous trophoblast and
et al.  showed that antiphospholipid syndrome (APS, a dis-
order of blood coagulation) is a major cause of early pregnancy
bodies. Patients with recurrent pregnancy loss have distinct
endometrial gene expression profiles when antiphospholipid
antibodies in circulation. These investigators speculated that
impaired endometrial differentiation and low DAF expression
before conception in these patients may compromise implanta-
tion and predispose to complement-mediated pregnancy failure.
Girardi et al.  showed in the mouse that antiphospholipid
antibodies cause complement activation and resulted in fetal
injury, but this was protected by concomitant treatment with
6. Trophoblast invasion of the stromal tissue and
The success of normal pregnancy depends on the protection
and growth of the semi-allogenic embryo within the mater-
nal uterine microenvironment. However, the mechanisms by
which the genetically incompatible embryo escapes maternal
immunological responses during early pregnancy need detailed
prior to implantation, and the loss of the uterine luminal epithe-
lium at the site of the implanting blastocyst make the embryo
more vulnerable to maternal insults. In the mouse and rat, and
ative at the maternal–conceptus interface to prevent the passage
of harmful molecules to the embryo. During the process of blas-
the luminal epithelium decidualize no sooner than the blasto-
cyst starts invading the luminal epithelial layer. This decidual
tissue immediately surrounding the implanting blastocyst (the
primary decidual zone: PDZ) is composed of several layers of
compact decidual cells and is avascular. This PDZ is character-
cells, and thus, called an epitheloid tissue. The incomplete tight
junctions serve as a filter to prevent passage of large molecules
such as immunoglobulins. The formation of PDZ appears to
be regulated by blastocyst trophoblast cells, since PDZ is not
formed in deciduoma that is formed by artificial stimulus or
difference between decidua and deciduoma is not only in tight
junctional molecules, but different distribution of lymphocyte
Progesterone and estrogen regulate synthesis of colony-
stimulation factor-1 (CSF-1) in the mouse uterus. A proper
increased the uterine CSF-1 concentration three times higher
than that of the control . This macrophage growth factor
is synthesized in high concentrations by the uterine epithelium
during pregnancy. Trophoblast cells express CSF-1 receptors.
Arceri et al.  found that CSF-1 mRNA was detectable in
the uterine epithelium just prior to implantation and increased
gradually to peak at days 14–15 of pregnancy. CSF-1 receptor
mRNA expression was observed in trophoblast cells and decid-
ual cells. These investigators felt that CSF-1 strongly implicate
in the regulation of placental growth and differentiation.
severe infertility and recurrent pregnancy loss due to defective
implantation [83,84]. Hoxa-10 mediates the progesterone-
stimulated proliferation of uterine stromal cells. Hoxa-10
panied by alterations in the expression of cyclin-dependent
kinase inhibitor genes. Hoxa-10 deficiency also leads to a
severe local immunological disturbance, characterized by a
polyclonal proliferation of T cells, that occurs in place of
the normal progesterone-mediated immunosuppression in the
peri-implantation uterus . When stromal cells decidual-
ize this tissue provides a special environment to NK cells for
their maturation. A study using mutant mice in IL-11 recep-
tor alpha chain gene demonstrated that IL-11 is essential for
decidualization . IL-11 has been shown to be essential for
providing this decidual cell environment . Popovici et al.
 studied interaction of human endometrium and trophoblast
included those for inflammatory response, immune response,
and chemotaxis. Hess et al.  studied paracrine communi-
cation between trophoblast and decidual cells and found that
trophoblast secretions modulate immune and angiogenic fac-
tors produced by decidual cells that may regulate trophoblast
differentiation, invasion, and placentation as well as leukocyte
recruitment, leukocyte proliferation, leukocyte activation, and
immunoacceptance of the fetal allograft. Pentraxin 3 is a mem-
ber of the pentraxin superfamily and plays an important role
in innate resistance against selected pathogens and in the reg-
ulation of inflammatory reactions. This secretory inflammatory
protein is up-regulated during the window of implantation in
human uterus , and in decidual cells produced by embry-
onic as well as artificial stimuli . Deletion of the pentraxin
3 gene results in failure in implantation and decidualization.
7. NK cells in pregnant uterus
family expressed in trophoblast cells coincident with establish-
the development of rodent placenta [93–95]. The role of PLP-A
is significant from the immunological point of view. Muller et
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
al.  demonstrated that PLP-A interacts with natural killer
(NK) lymphocytes and binds their surface marker glycopro-
tein, gp42. The histological studies showed that this binding
overlying the developing chorioallantoic placenta. This binding
in the mesometrial decidual tissue was first observed on day
9 of rat pregnancy and radiated dramatically from developing
trophoblast cells and was prominently associated with mesome-
trial vasculature. The distribution of the PLP-A binding to pg42
reflects the distribution of NK cells in the decidual tissue. The
binding of PLP-A to NK cells can inhibit the catalytic activities
of rat NK cells. A population of NK cells within the mesome-
trial compartment of the pregnant uterus is considered to be
regulated by trophoblast-secreted PLP-A, thus, avoiding elimi-
nation of the growing embryo by the maternal immune system.
Another member of prolactin-like protein family (PLP-N) was
localized in endovascular and interstitial migratory trophoblast
et al.  showed that human decidual NK cells, different from
NK cells from peripheral blood, have unique ability to regulate
trophoblast invasion by secreting growth factors and cytokines.
reviewed by Croy et al. . IL-15 is essential for the support
of NK cell differentiation in the decidualizing uterus [99,100].
In human pregnancy HLA-E molecule is expressed on the tro-
phoblast cell surface. HLA-E acts as a sentinel molecule on
NK attack. HLA-E on trophoblast cell surface interacts with its
receptor on uterine NK cells and may regulate other functions
besides cytolysis during implantation .
The mesometrial triangle is a specialized region on the
mesometrial side of each placenta in the rodent uterus. This
area is important for supply of rich vascularization for full func-
tion of the discoidal placenta during pregnancy. The arteries in
ies, dilate markedly and follow tortuous course to supply blood
to the decidua basalis. Rodent arteries in this region are con-
sidered equivalent to the human spiral arteries . Invasion
of spiral arteries by trophoblast cells is considered as the final
stage of implantation, and, thus, relationship between implanta-
tion process and immune cells in this region of the uterus during
early pregnancy needs to be discussed. Starting on day 6 of
mouse pregnancy there begins accumulation of lymphocytes in
the mesometrial triangle. The accumulation continues to reach
at maximum at midgestation. This aggregation of lymphocytes
becomes prominent as pregnancy progresses and lymphocytes
are granulated. This aggregate was used to be called “metrial
cells originate by local activation of uterine NK cells. Coinci-
dent with blastocyst implantation and decidualization, uterine
NK cells become activated; they proliferate and produce inter-
The presence of NK cells in this area is essential for robust
decidualization. Cytokine IL-15 plays an important role in dif-
ferentiation of NK cells. Guimond et al.  demonstrated
that these NK cells play an essential role in proper formation
of the vascular system necessary for maintenance of pregnancy
Fig. 1. To understand the process of blastocyst implantation, we need to study
functions of endocrine cells, endometrium, embryo (trophoblast), and immune
are produced by endocrine cells, endometrium and trophoblast cells. BIEFs are
considered not only to facilitate implantation of blastocyst, but also to modulate
immune cells to tolerate (semi)-allogenic embryo.
because deletion of these NK cells resulted in severe compro-
mise of pregnancy. At this stage of pregnancy immune cells in
this special region of the uterus indeed collaborate with other
implantation essential factors for maintenance of pregnancy.
Erlebacher et al.  studied as to how the maternal T
cells respond to fetal antigens. Based on their experimental
data, they found that maternal T cells become aware of the
fetal allograft exclusively via “indirect” antigen presentation,
that is, T cell engagement requires the uptake and process-
ing of fetal/placental antigen by maternal antigen presenting
cells (APCs). These authors demonstrated that there are three
mechanisms in tandem that protect implanting embryo from
rejection by the maternal immune system: (1) T cells recog-
presentation by maternal antigen presenting cells (APCs), in
be activated by embryonic antigens are clonally deleted before
these T cells are primed to be toxic to embryo, and (3) inadver-
tently activated CD8+ T cells are still unable to attack the fetus.
Additionally, these authors found that antigen presentation by
APCs may not occur during the process of implantation as anti-
gen presentation was only detectable after invasion of maternal
spiral arteries by trophoblast cells.
8. Future directions
In this review I tried to point out the factors that are essen-
tial for blastocyst implantation (BIEFs) need to be studied for
their immunological aspect as well. To investigate the pro-
cess of blastocyst implantation the minimal constituents we
have to look at the same time are: endocrine organ products
(hormones), endometrium, embryo (trophoblast), BIEFs, and
of protein and steroid hormones acting on uterine tissues. Their
effects are frequently mediated via growth factors, cytokines
K. Yoshinaga / Seminars in Cell & Developmental Biology 19 (2008) 161–169
and other molecules. These molecules, supportive of embryonic
onic/placental tissues, but also on the maternal immune system.
Since the target of supporting molecules for embryonic devel-
opment include evasion of the hostile maternal immune system,
clarification of the blastocyst implantation process must include
clarification of the embryo–uterine interactions as well as the
process of evasion of the hostile maternal immune system.
ethical considerations and availability of study samples. There
is a huge species variation in the mode of implantation, thus,
the results obtained in one species may not be immediately
applicable to another. There are large gaps in method and fre-
quency of collecting data depending on the research discipline.
To overcome these handicaps in implantation research, an inter-
disciplinary team research approach is strongly recommended.
The current state of knowledge of implantation is not suf-
ficiently mature enough to develop strategies for alleviation of
infertility in women due to implantation failure. It is hoped that
further investigation in this area will provide new information
that will restore fertility potential in women with implantation-
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